Vehicle-to-everything (V2X) communications, in particular vehicle-to-vehicle (V2V) communications, require reliable communication links toward all entities (e.g. vehicles) in proximity of a given vehicle. A known single antenna (numbered 100) is shown schematically in FIG. 1. The antenna includes a vehicular communication modem (802.11p MAC/PHY) 102 and an Ethernet interface (I/F) 104. Data arriving from an Ethernet interface from the protocol stack software layer (not shown) is transmitted over the air using modem 102, and vice versa.
In vehicles with a single antenna, the antenna may be at times blocked and/or fails to provide omnidirectional coverage. In such and other cases, some vehicles may have two antennas and therefore use V2X communication scenarios involving two antennas. The two antennas may be “remote antennas” in the sense that they located far from each other (and not in the same enclosure). For example, two remote antennas may be separated by a distance of 0.5 meter or more. In some cases, two remote antennas may be located in trucks or large vehicles. In other cases, two remote antennas may be hidden for styling reasons. FIG. 2A shows a truck 200 with two remote antennas 202 and 204 positioned at, respectively, the front and the rear of the truck. Typically, in known two remote antenna configurations, the two antennas are connected via a coaxial (Coax) cable to a centralized modem (i.e. a vehicular communication modem such as modem 102 above. However, Coax cables are expensive, have significant weight, and are hard to handle because of their limited bending radius. In addition, the amplification circuitry required to compensate Coax attenuation has inherent cost.
Digital connectivity is cheaper than Coax cabling since it can use a plain twisted pair cable. Cabling weight is lower, and there are no bending radius limitations. However, this comes with a penalty. Digital connectivity requires reception circuitry at each antenna, for translation of a modulated radio frequency (RF) signal to a digital signal. Digital connectivity may exchange modulated data such as IQ (in-phase and quadrature) samples. However, while modulated data connectivity may provide optimal diversity, signal bandwidth needs to be high and latency needs to be low. These requirements are hard to achieve, complicate the design, and (if existing at all) would be very expensive.
Another option with a two-antenna configuration is to exchange un-modulated data, i.e. decoded bits. At a receive side, the exchange of decoded bits would be trivial, at the expense of losing noise gain (3 dB) but benefiting from array gain (i.e. from the gain resulting from different antenna placements). At the transmit side, the challenge is to synchronize the start time of the two antennas, under the assumption that transmission from a single antenna will not meet the requirement of transmitting the signal to a required distance (for example 400 m) in any direction around the vehicle. The transmission start decision may be affected by information existing at only one antenna. For example, Clear Channel Assessment (CCA) may be asserted at one antenna but not asserted at the other. For example, exchanges of CCA status may be subject to latency.
The digital connectivity between the two antennas may be indirect, through switches. However, a private digital bus is not common in vehicles. Each switch adds latency, which is a great concern for the modulated data exchange or the updated CCA data.
There is therefore a need for and it would be advantageous to have apparatus and/or methods that overcome the abovementioned limitations for having digital connectivity that exchanges un-modulated data (or “un-modulated data signals”) between two remote antennas in a vehicle in V2X communications.